As the oil and gas industry is developed, unconventional gas reservoirs (tight sandstone gas, coal-bed gas, and shale gas) have become a new developing trend of the global low-carbon economy, and their exploration and development has been scaled up gradually. Among proved oil reserves in China, low-permeability oilfields account for one quarter or higher share. As most oil fields enter into a high water-cut stage, the exploitation becomes more and more difficult. Exploiting those oil and gas fields is of far-reaching importance for maintaining stability of the oil energy resources in China.
Fracturing techniques have been widely applied as conventional stimulation and stimulated injection techniques for oil and gas fields. Fracturing fluids are one of the keys in fracturing operation, and are often referred to as fracturing “blood”. Fracturing fluids that are applied the most widely at present are slickwater fracturing fluids and guanidine gel fracturing fluids. However, slickwater fracturing fluids have low viscosity and limited solid carrying capacity; guanidine gel fracturing fluids may cause severe damages to the reservoirs. Clean fracturing fluids have outstanding performance in actual oil and gas mining, and have received extensive attention from domestic and foreign researchers and have been applied gradually owing to a series of advantages, including simple molecular structure, high dissolvability, fewer damages to reservoirs, and good viscosity breaking performance, etc. However, for unconventional oil and gas reservoirs, the demand for fracturing fluid and water resource is high owing to great well depth and long horizontal section, and the flow-back fluid generated after fracturing not only involves high treatment cost, but also may result in severe environmental pollution if it is discharged to the external environment. Moreover, owing to the fact that the molecular structure of slickwater fracturing fluid or guanidine gel fracturing fluid is changed or destroyed after gel breaking, complex chemical and physical treatment is required or a large quantity of original fluid has to be added to the flow-back fluid, before the flow-back fluid can be reused. Therefore, how to efficiently utilize the large quantity of flow-back fluid is a challenge in efficient fracturing stimulation of unconventional oil and gas reservoirs at present. Reuse of flow-back fluid is a future developing trend of the oil and gas field industry, in consideration of the cost and environmental protection aspects.